Re: [boost] none.hpp

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2006-07-25 04:01:15

Martin Wille <mw8329_at_[hidden]> writes:

> Joe Gottman wrote:
>> In order to avoid having an unnamed namespace inside a header file, none.hpp 
>> has been rewritten as
>> 
>>     namespace boost {
>>         none_t const none = ((none_t)0) ;
>>     } // namespace
>> 
>> But if two source files contain none.hpp won't this violate the 
>> one-definition rule?
>
> none would have internal linkage according to 7.1.5.1/2. So it wouldn't
> violate the ODR. But a pointer to none from one translation unit would
> differ from a pointer to none from a different TU.

How about the attached?

The use of an inline function avoids the ODR violation, and the use of the
detail::none_helper type with private constructors as the parameter ensures
that noone can actually call the function, since they cannot construct a
none_helper to pass. Likewise, they cannot define another function for use as
none_t, since that function cannot construct the return value.

Anthony
-- 
Anthony Williams
Software Developer
Just Software Solutions Ltd
http://www.justsoftwaresolutions.co.uk